1. Field of the Invention
This invention relates to a circuit for indicating the quality of an audio signal received from a signal transmission path and more particularly to use of this circuit in a system for selecting one of a plurality of signal transmission paths for connection to a utilization device.
2. Description of the Prior Art
In many radio communication systems a radio receiver is installed at each of several locations so that signals from a mobile radio transmitter can be reliably received from all locations of the transmitter. The receivers are connected by signal transmission paths to a common location. There will be many locations of the transmitter from which two or more of the receivers receive signals from the transmitter and produce a usable audio signal output. Under such conditions, only the highest quality audio signals should be utilized so as to avoid interference or distortion.
U.S. Pat. No. 3,729,682, which was issued to the assignee of the present invention, discloses circuitry for indicating the quality of an audio signal received from a radio receiver, comparing the indicated qualities of a plurality of such received signals and selecting the best audio signal for utilization. Although the circuitry disclosed in the U.S. Pat. No. 3,729,682 generally performs its intended function of selecting the highest quality audio signal, it sometimes has difficulty in discriminating between a noise signal having audio characteristics and a valid audio signal. For example, in a system utilizing telephone lines as the signal transmission paths, a failed telephone line or a line connected to a failed receiver often carries noise signals, such as low level cross-talk voltages, which have characteristics similar to a valid audio signal. Although peak-to-peak amplitudes of envelopes of these noise signals are usually smaller than such amplitudes of valid audio signals, the noise signals are sometimes selected for connection. This is because the circuitry determines signal quality solely from the amplitude of the minimum signal excursions. This amplitude is equated to the noise content of a received signal and selection is made without regard to the amplitude of maximum signal excursions.
Additionally, effective operation of the quality indicating circuitry disclosed in the U.S. Pat. No. 3,729,682 is complicated by a necessity for initial adjustments to compensate for non-uniform gain characteristics of the signal transmission paths connecting the receivers to the circuitry. Also, since the circuitry does not automatically adjust to changes in the gains of the paths occurring after the initial adjustments, such changes sometimes cause the circuitry to give incorrect indications of the relative qualities of valid audio signals. For example, a first signal having a high noise content and therefore low quality relative to a second signal might be selected for utilization if the transmission path for the first signal suffered a change which reduced its gain relative to the transmission path for the second signal. The erroneous selection would occur if said change caused the magnitude of the first signal to be reduced to a level such that the amplitude of the minimum excursions of the envelope of the first signal is less than the corresponding amplitude for the second signal.
The present quality indicating circuit produces an indication of signal quality which ensures selection of the highest quality audio signal regardless of the presence of noise signals having audio characteristics. The circuit produces a quality signal having a magnitude which is a logarithmic representation of the signal-to-noise ratio of a monitored audio signal. This quality signal is developed by determining the difference between the amplitudes of maximum and minimum excursions of an envelope signal formed from a signal which is a logarithmic representation of the monitored audio signal. Since this difference is independent of absolute signal levels and is typically smaller for noise signals than for valid signals, discrimination between a valid audio signal and a low level noise signal having audio characteristics is easily accomplished. Also, because of the quality signal independence from absolute signal magnitudes, no circuitry adjustment is necessary to compensate for non-uniform gain characteristics of the signal transmission paths and selection of the highest quality audio signal is effected regardless of changes in gains of the paths.
A monitor which forms an envelope having maximum and minimum amplitudes indicative of maximum audio signal level and noise level, respectively, is disclosed in U.S. Pat. No. 3,238,457. The monitor makes a comparison between maximum and minimum amplitudes of the envelope, but a phase inversion technique not required in the present invention is utilized to determine the minimum amplitude. Also, no logarithmic conversion is provided and the monitor does not produce a signal which is representative of the signal-to-noise ratio of the audio signal. The monitor only gives an indication when the difference between the maximum and minimum amplitudes of the envelope exceeds a predetermined magnitude. Thus, no signal having a magnitude representative of audio signal quality is produced for comparison.